Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"We said: 'O Messenger of Allah! We know what it means to send greetings upon you, but what does it mean to send peace and blessings upon you?' He said: 'Say: "Allahumma salli 'ala Muhammad (SAW)in 'abdika wa Rasulika kama salayta 'ala Ibrahima, wa barik 'ala Muhammad (SAW) (wa 'ala ali Muhammad (SAW)in) kama barakta 'ala Ibrahima [O Allah, send Your grace, honor and mercy upon Muhammad (SAW), Your slave and Messenger, as You sent Your (grace, honour and mercy) upon Ibrahim, and send Your blessings upon Muhammad (SAW) (and the family of Muhammad (SAW)) as You sent Your blessings upon Ibrahim]."